In the use of an element having a chamber, it is desirable to produce residual compressive stresses in metal of the element for reducing the effects of tensile stresses on the element during use of said element. The invention relates to a method for treating the metal of the element in order to produce desirable compressive stresses in the metal of the element for increasing fatigue life under loading of said element.
For example, a boom of an excavator is generally a welded box beam structure having an interior chamber. The box beam structure is constructed of four side walls each welded one to the other with fillet welds or butt welds at the corners of the structure. Such a boom construction is disclosed, for example, in U.S. Pat. No. 3,882,654 which issued to Yancey on May 13, 1975.
During use of the excavator, loads are engaged and carried by a bucket or shovel implement. A result is to place the boom of the excavator under torsional loading. Continual torsional loading, especially under harsh operating conditions, can result in fatigue cracks in the boom structure and subsequent failure of the boom. The resultant necessary replacement of the boom represents a considerable expense of time, labor, and material.
Torsional loading creates tensile loads at the weld roots which can eventually lead to fatigue cracks in roots of the welds of the boom. Placing the welds under residual compressive stresses tends to counter the effect of subsequent tensile stresses at the same location. The boom can then withstand a higher number of torsional loadings without fatigue cracks appearing.
Therefore, it is desirable to provide a method for treating the metal of an element having a chamber in order to produce residual compressive stresses in the metal of the element for increasing fatigue life of said element which is subjected to tensile stresses.